SENSORY STRUCTURES. 311 



to know. But these statements imply forgetfulness of the 

 relative size of brain to body, and tend moreover to exagger- 

 ate the importance of mere size. The complexity of a brain 

 is the important fact, not its size, and there is no doubt that 

 the cleverer insects (ants, bees, and wasps), have more com- 

 plex brains than the others. As in other Arthropods, the 

 nervous system consists (a) of a dorsal brain or supra- 

 cesophageal ganglionic mass, and (fr) of a double ventral 

 nerve cord with a number of paired ganglia of which the 

 most anterior (the sub-cesophageal) are linked to the brain 

 by a ring commissure around the gullet, and (c) of nerves 

 given off from the various ganglia to the sense organs, the 

 alimentary canal, and the other organs. In many of the 

 higher insects the ganglia of the ventral nerve cord are in 

 some degree concentrated, and the adults are usually more 

 centralised than the larvae. 



Sensory Structures. 



Animals so much alive as Insects, and in surroundings so 

 stimulating as many of them enjoy, have naturally highly 

 developed sense organs. 



Two compound eyes are present on the head of all adults 

 except the primitive Collembola, the degenerate lice, the 

 likewise parasitic fleas, and blind insects which live in caves 

 or other dark places. Each eye contains a large number of 

 similar elements, in each of which we can distinguish (i) a 

 cuticular or corneal facet; (2) a glassy lens-like portion; (3) a 

 retinal portion in association with which are fibres from the 

 optic nerve, and there are also pigmented cells between the 

 elements (cf. p. 259). 



Simple eyes or ocelli are present in addition to the com- 

 pound eyes in the adults of many insects, e.g., ants, bees, 

 and wasps ; they occur without the accompaniment of com- 

 pound eyes in Collembola, lice, and fleas, and they are 

 usually the only eyes possessed by larvae. They have only 

 one lens (monomeniscous), whereas the compound forms 

 have many lenses (polymeniscous). Their structure varies 

 greatly, and their use is very uncertain. 



Auditory (or chordotonal) organs have been found in all orders of 

 Insects (except as yet the Thysanoptera), and occur both in the larvae and 

 in the adults. Their essential structure is as follows : A nerve ends in 



